ABSTRACT

The Upper Cretaceous Niobrara Formation in the Denver-Julesburg (D-J) Basin, United States, records late Turonian–early Campanian transgressive–regressive cycles that governed the rhythmic character of pelagic and detrital sedimentation along the former carbonate ramp of the Western Interior seaway (WIS). The general stratigraphic and architectural complexity of the full Niobrara section has long been recognized; however, fine-scale subdivision of the formation from a sequence stratigraphic aspect has been lacking and is critical for basin-scale analysis. The timing and controls of significant changes in architectural and sediment-accumulation patterns have important implications for the Niobrara petroleum system. As part of this study, we present a sequence stratigraphic framework appropriate for the distal carbonate ramp of the WIS. We project published biozone mapping and radiometric ages into basin-wide well control to establish the timing of chronostratigraphic surfaces. Age-constrained isochore maps reveal a chronology of dynamic paleoseafloor morphology that is critical to understanding the stratigraphic and architectural evolution of the Niobrara.

At a regional scale, thickness patterns suggest that the north-south–oriented flexural forebulge in the Sevier foreland basin had migrated eastward to the position of the future Rocky Mountains, a location coincident with the western edge of dominantly pelagic carbonate deposition in the backbulge of the WIS basin. At the D-J Basin scale, chronostratigraphic isochore maps of four third-order sequences, and their finer-scale subintervals, record two broadly distinct periods of depositional influence in the Niobrara. Between ca. 90.0 and ca. 85.7 Ma (upper Turonian–lower Santonian), relatively even deposition was dominated by a pattern of differential sediment accumulation. Systematic reversals of thicks and thins through time are indicative of compensational infilling patterns with no influence of tectonic uplifts along the seafloor. In contrast, from ca. 85.7 to ca. 81.7 Ma (lower Santonian–lower Campanian), a reorganization of earlier architecture was driven by the influence of sublinear uplifts along the trend of the emergent Transcontinental arch. As a result, sediment accumulation was dominated by patterns of draping over the long-lived paleohighs. Absolute timing of the architectural changes of the Niobrara suggests a link among Sevier thrusting events, a migrating flexural forebulge, and uplifts at reactivated basement shear zones in the distal foreland. Thus, both active tectonics and the inherited Proterozoic basement fabric of the WIS influenced the evolution of the Niobrara Formation. This example from the Cretaceous WIS shows that age-constraining sequence stratigraphy for regional isochore mapping can reveal relationships among the structural response to far-field tectonic events, oceanographic variations, and stratigraphic architecture.

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